I. Field of the Invention
The present invention relates to Code Division Multiple Access (CDMA) systems. More particularly, the present invention includes, but is not limited to, a novel and improved CDMA base station that performs various combinations of the following: 1) decresting CDMA signal peaks, 2) shaping the in-band frequency spectrum of CDMA signals, 3) generating a ratio of in-band to out-of-band signal strength, and/or 4) controlling transmit power based on quadrature signal calculations.
II. Description of the Related Art
Code Division Multiple Access (CDMA) technology is commonly used in communications systems. In a typical CDMA system, a CDMA base station transmits a CDMA signal to numerous CDMA communications devices, such as wireless telephones. The CDMA signal is comprised of numerous individual user signals. The CDMA base station generates the CDMA signal by encoding each individual user signal with a unique spreading sequence, such as a pseudo random sequence. The CDMA base station then adds the encoded user signals together to form the CDMA signal.
In a CDMA system, individual user signals are not separated based on frequency or time, but are spread across the entire frequency band. Each CDMA communications device derives its particular user signal based on the unique spreading sequence. Due to this combination of multiple signals encoded with random sequences, the CDMA signal has random signal peaks that cause problems when the CDMA signal is amplified. In contrast, non-CDMA signals do not typically have such random characteristics. For example, a frequency modulated signal fits within a constant signal envelope because individual user signals are placed within discreet frequency bands and are not combined or encoded with random sequences.
CDMA signal transmission has special power concerns because the CDMA signals are spread across the frequency band. Since the CDMA signals share the frequency band, each signal represents noise to the other signals. Thus, CDMA transmission systems must carefully track the power of each signal.
Baseband CDMA signals are typically generated in a well-known quadrature format comprised of quadrature CDMA signals I and Q. Quadrature CDMA signals I and Q are transmitted using carriers of the same frequency, but in phase quadrature. In other words, an RF CDMA signal can be constructed by modulating I by cosine (2xc3x97pixc3x97frequencyxc3x97time) and by modulating Q by sine (2xc3x97pixc3x97frequencyxc3x97time). In IS-95A, quadrature signals carry the same data with different pseudo-random sequence codes.
FIG. 1 illustrates an ideal frequency spectrum of a typical CDMA signal. The vertical axis represents signal power, and the horizontal axis represents frequency. The desired in-band signal power is contained within the bandwidth defined by corner frequencies around a center frequency. A typical example is a 1.25 MHz bandwidth centered about a 1.96 GHz center frequency with corner frequencies at (1.96 GHzxe2x88x92625 KHz) and (1.96 GHz+625 KHz). The signal power drops significantly outside of the bandwidth, but some undesired out-of-band signal power is still present and is shaded on FIG. 1. Out-of band signal power is undesirable because it represents wasted power that interferes with other signals in neighboring frequency bands.
FIG. 2 illustrates a time domain plot of a typical CDMA signal. The vertical axis represents CDMA signal amplitude in volts, and the horizontal axis represents time. The dashed lines represent a maximum positive signal voltage (+Vmax) above the zero voltage point, and a negative maximum signal voltage (xe2x88x92Vmax) below the zero voltage point. The CDMA signal has xe2x80x9cpeaksxe2x80x9d above and below the Vmax voltages. The peaks are shaded on FIG. 2.
FIG. 3 illustrates the operating characteristics of a typical power amplifier used to amplify a CDMA signal. The horizontal axis represents the input signal power (Pin), and the vertical axis represents the output signal power (Pout). If Pin is below a maximum power level (Pmax), then the power amplifier operates in a linear manner where an increase in Pin is matched by a proportional increase in Pout. If Pin is above Pmax, then the power amplifier operates in a nonlinear manner where an increase in Pin is not matched by a proportional increase in Pout. Pout is less than ideal in the nonlinear operating range.
It should be noted that the Vmax voltage levels on FIG. 2 correspond to the Pmax on FIG. 3. Thus, the random signal peaks above +Vmax and below and xe2x88x92Vmax drive the power amplifier above Pmax into the nonlinear operating range. When operated in the nonlinear range, the power amplifier exhibits undesirable performance in the form of decreased fidelity and increased noise. In contrast, the typical Frequency Modulated (FM) signal does not have random signal peaks, so the power amplifier is able to continuously operate below the maximum power level.
The power amplifier generates additional out-of-band signal power when operated in the nonlinear range. Out-of-band signal power is a problem because it interferes with other signals in the neighboring frequency bands. Government agencies, such as the Federal Communications Commission in the United States, strictly regulate the interference caused. by out-of-band signal power.
An existing solution to the problem is implemented during base station testing. Test equipment is used to calculate a ratio for a test CDMA signal transmitted by the base station. The ratio represents the in-band signal power versus the out-of-band signal power. The base station transmit power is adjusted during the testing so the ratio is below a maximum value with a margin for some ratio increase under the maximum value. This usually Unfortunately, the ratio is not calculated and is not used during normal base station operation in the field. Test equipment is used to calculate the ratio, and base stations are not equipped to calculate the ratio in the field. Thus, the ratio is not automatically generated and used to control operation in the field where changes in temperature and load alter base station operation.
Another existing solution to this problem is to operate the CDMA base station so a ratio of the power out to the pilot signal does not exceed a value, such as five. This solution is lacking because a maximum power level based on the pilot signal is not an optimal estimate of the point where out-of-band signal power becomes a problem. As a result, the range and capacity of the base station is not optimized.
FIG. 4 depicts a multi-sector base station 1100 that is currently known in the art. The base station 1100 is divided into geographic sectors with callers A-F in sector 1 and callers G-L in sector 2. For the sake of illustration, caller F will move from sector 1 to sector 2 as indicated by the dashed lines, but the operation of the base station 1100 is first discussed prior to the caller F move from sector 1 to sector 2. Those skilled in the art will appreciate that the diagram of the base station 1100 has been simplified for clarity.
The sector 1 portion of the base station 1100 includes cell site modems 1102 and 1104, gain control 1106, summing circuit 1108, CDMA signal processor 1110 including gain 1112, and antenna 1114. The sector 2 portion of the base station 1100 includes cell site modems 1122 and 1124, gain control 1126, summing circuit 1128, CDMA signal processor 1130 including gain 1132, and antenna 1134.
In operation, the cell site modem 1102 receives signals for callers A, B, C and applies conventional CDMA processing to generate CDMA quadrature signals I and Q. The cell site modem 1102 provides the CDMA quadrature signals I and Q to the summing circuit 1108. The cell site modem 1104 receives signals for callers D, E, F and applies conventional CDMA processing to generate CDMA quadrature signals I and Q. The cell site modem 1104 provides the CDMA quadrature signals I and Q to the summing circuit 1108. The summing circuit separately combines the I signals and the Q signals and transfers them to the CDMA signal processor 1110. The CDMA signal processor 1110 performs analog conversion, filtering, up-conversion, and amplification to provide a Radio Frequency (RF) CDMA signal to the antenna 1114. The antenna 1114 transmits the RF CDMA signal 1116 over the air to the callers A-F in sector 1.
The cell site modem 1122 receives signals for callers G, H, I and applies conventional CDMA processing to generate CDMA quadrature signals I and Q. The cell site modem 1122 provides the CDMA quadrature signals I and Q to the summing circuit 1128. The cell site modem 1124 receives signals for callers J, K, L and applies conventional CDMA processing to generate CDMA quadrature signals I and Q. The cell site modem 1124 provides the CDMA quadrature signals I and Q to the summing circuit 1128. The summing circuit separately combines the I signals and the Q signals and transfers them to the CDMA signal processor 1130. The CDMA signal processor 1130 performs analog conversion, filtering, up-conversion, and amplification to provide an RF CDMA signal to the antenna 1134. The antenna 1134 transmits the RF CDMA signal 1136 over the air to the callers G-L in sector 2.
Each cell cite modem 1102, 1104, 1122, and 1124 provides gain information 1118 to both the gain control 1106 and the gain control 1126. The gain information 1118 includes the squared gain for each call, pilot signal, and overhead. Gain control 1106 and gain control 1126 each maintain a database that incorporates the gain information 1118.
The CDMA signal processor 1110 monitors the transmit power (Pout) of the CDMA signal 1116 for sector 1 and provides a Pout value 1119 for sector 1 to the gain control 1106. The gain control 1106 compares the Pout value 1119 for the CDMA signal 1116 to a Gain Value (GV) equal to the sum of the squared gains for the CDMA signal 1116. The squared gains for the CDMA signal 1116 are obtained from the gain information 1118. The gain control 1106 transfers a control signal 1117 to the gain 1112 to adjust the Pout to maintain a ratio of GV to Pout at a pre-determined value.
FIG. 5 shows the desired relationship between Pout and the GV. The points X and Y represent operational measurements, and the arrows represent the control applied through the control signal 1117 to the gain 1112 to maintain the pre-determined value. Those skilled in the art are aware that the slope of the pre-determined value blossoms during start-up and wilts during shut-down.
On FIG. 4, the CDMA signal processor 1130 monitors the Pout for sector 2 and provides the Pout value 1139 for sector 2 to the gain control 1126. The gain control 1126 compares the Pout value 1139 for the CDMA signal 1136 to a GV equal to the sum of the squared gains for the CDMA signal 1136. The squared gains for the CDMA signal 1136 are obtained from the gain information 1118. The gain control 1126 transfers a control signal 1137 to the gain 1132 to adjust the Pout to maintain a ratio of GV to Pout at a pre-determined value.
When caller F moves from sector 1 to sector 2, the cell site modem 1104 or sector 1 transfers the caller F quadrature signals 1141 and 1142 to the summing circuit 1128 for sector 2. Thus, the CDMA signal 1136 now includes the caller F signal. As a result, the gain control 1126 must now add the square of the caller F gain to its GV.
It should be appreciated that each cell site modem must transfer all gain information 1118 to the gain control in each sector. This requires a data transfer arrangement across all sectors, and much of the transferred data is unnecessary. For example, gain control 1126 does not need the gain for caller A unless caller A moves into sector 2. The gain control for each sector must also track the calls in its sector and perform repeated calculations based on a changing database.
CDMA systems would be improved by techniques to reduce the noise contribution of the power amplifier in the base station. The noise reduction would directly increase the power and efficiency of the CDMA base station. CDMA systems would also be improved through transmission at a power level just below the point where out-of-band signal power becomes a problem. Transmission at this power level would optimize the range and capacity of the base station. In addition, the current power calculation technique for CDMA base stations should be improved to reduce data transfer and storage.
The above-described problems are solved with CDMA transmission control technology. This technology can include decresting logic that reduces or eliminates random peaks in the CDMA signal. The power amplifier in a CDMA base station can then operate at increased power levels without exceeding out-of-band signal power limitations. Testing has shown a base station power increase of 3dB when decresting technology is used.
The decresting logic generates a correction signal in response to peaks in the CDMA signal that exceed a threshold. The threshold typically corresponds to the maximum power level of a power amplifier. The decresting logic combines the correction signal with the CDMA signal to generate a decrested CDMA signal with reduced peaks. In some examples of the invention, the decresting logic processes polar coordinate representations of the quadrature components of the CDMA signal to generate the correction signal.
The transmission control technology can include spectral shaping logic that reduces the out-of-band signal power in the CDMA signal. The spectral shaping logic attenuates the in-band CDMA signal near the corner frequencies to reduce components that provide a disproportionate contribution to out-of-band signal power. The power amplifier in the CDMA base station can then operate at higher power levels without exceeding out-of-band signal power limitations.
The transmission control technology can include ratio logic that allows a CDMA base station to operate at an optimized power level without generating improper amounts of out-of-band noise. Ratio logic automatically generates a ratio of the CDMA signal strength of the in-band components versus the out-of-band components to eliminate the need use pre-set margins for ratio increases in the field. In some examples of the invention, the ratio logic uses the ratios to generate metric signals that indicate if transmit power should be limited and that indicate excess forward link capacity. In some examples of the invention, the ratio logic uses the ratios to set the decresting threshold.
The transmission control technology can include power control logic that controls the transmit power of the CDMA signal. The power control logic eliminates unnecessary data transfer and storage because gain control is accomplished without transferring or using per call gain information. The power control logic generates a ratio based on the power of the transmitted signal and a power value generated from quadrature components of the CDMA signal. The power control logic generates a power control signal based on the ratio. In some examples of the invention, the decresting logic provides the power value.
The transmission control technology causes a CDMA base station to operate more efficiently. The transmission control technology also causes the CDMA base station to operate with a greater range or capacity. This improvement is passed on to the wireless communications user in the form of higher quality and lower cost.